Verisign has dramatically slashed its estimates for the number of domains in existence in its quarterly Domain Name Industry Brief reports, two of which were published this week.

The headline number for the end of the fourth quarter is 329.3 million, a 0.7% increase sequentially and a 6.8% increase annually.

But it’s actually a lower number than Verisign reported in its second-quarter report just five months ago, which was 334.6 million.

The big swinger, as you may have guessed if you track this kind of thing, was .tk, the Freenom ccTLD where names are given away for free and then reclaimed and parked by the registry when they are deleted for abuse expire.

It seems a change in the way .tk is counted (or estimated) is the cause of the dip.

Verisign gets its gTLD data for the report from ICANN-published zone files and its ccTLD data from independent researcher Zooknic.

Problem is, Zook hasn’t had up-to-date data on .tk for a couple of years, so every DNIB published since then has been based on its December 2014 numbers.

But with the Q3 report (pdf), Zook revised its .tk estimates down by about six million names.

In earlier reports, the ccTLD was being reported at about 25 million names (exact numbers were not given), but now that’s been slashed to 18.7 million, relegating it to the second-largest ccTLD after China’s .cn, which has 21.1 million.

I’ve asked Freenom to confirm the latest numbers are correct and will update this post if I get a response.

Verisign does not say what caused the decision to scale down .tk’s numbers, but explains what happened like this:

In Q3 2016, Zooknic reported a significant decline in the .tk zone and restated the estimated zone size of .tk for each quarter from Q4 2014 through Q3 2016 using a proprietary methodology. As a result, for comparative purposes of this DNIB to the Q3 2016 DNIB and the Q4 2015 DNIB, Verisign has applied an updated estimate of the total zone size across all TLDs for Q3 2016 of 327.0 million and Q4 2015 of 307.7 million and an updated estimate of the total ccTLD zone size for Q3 2016 of 140.1 million and Q4 2015 of 138.1 million.

Apples-to-apples comparisons in the Q4 report show the ccTLD universe was up to 142.7 million names, a 1.8% sequential increase and up 3.1% on 2015. Excluding .tk, annual growth was 6.9%.

Verisign’s own .com and .net combined grew 1.7% to 142.2 million names at the end of the year, one percentage point smaller than their 2015 growth.

The .tk domain is now the biggest ccTLD in the world, according to the latest stats from Centr.

In its just-published biannual Domainwire Stat Report, Centr says that .tk had 16.7 million registered domains in April, taking the #1 spot in the league table for the first time.

It now out-ranks Germany’s .de (15.4 million), the United Kingdom’s .uk (10.5 million), China’s .cn (7.5 million) and the Netherland’s .nl (5.2 million), despite Tokelau having a population of less than 1,500.

The reason for its success, of course, is that .tk domains are free to register. The ccTLD frequently pops up towards the top of abuse lists for that very reason.

At 54.7%, .tk wasn’t the fastest-growing ccTLD over the six months covered by Centr’s report, however. That honor belongs to .cn, which bounced back from previous declines with an 82.7% growth rate.

As a result, it’s also regularly recognized by the Anti-Phishing Working Group as one of the most-abused TLDs out there, though the company says its business model allows it turn off abusive domains at will.